Mobile radio communication systems provide for two way radio communications between a central control station and a fleet of cars, trucks, or other vehicles. Typical users of mobile radio communication systems include, for example, police departments, fire departments, taxi services and delivery services. Present mobile radio communication systems can be configured to provide for radio communications between the control station and all vehicles in a fleet, between the control station and selected vehicles in the fleet, or between different individual vehicles or groups of vehicles within a fleet.
Conventional mobile radio communication systems are typically organized with a number of vehicles and a control station assigned to a single common channel for a given coverage zone. A user assigned to the common channel must wait until no one else is transmitting on that channel before sending a message, because only one transmission at a time can be processed by a single channel. Even when a transmission is addressed to only one user in a conventional system (and therefor not heard by other users), the other users in the system must wait until that transmission is completed before they can use the system to communicate.
Mobile radio communication systems using transmission trunking are an improvement over conventional mobile radio communication systems in that trunked systems enable two or more users to communicate on the system at the same time. For instance, the dispatch console in a transmission trunked communication system can be communicating with one of the vehicles in the fleet; while, at the same time, two other vehicles in the fleet can be using the same trunked system to communicate with each other. Transmission trunked communication systems group a number of channels for the same coverage zone together into a single radio communication system, with each channel accessible to each user in the system. Because each user will only need to communicate over the trunked system part of the time, and because it is unlikely that all users will need to communicate at precisely the same time, the number of channels assigned to a trunked system group can always be less than the number of users allocated to that trunked system group.
The United States Federal Communications Commission (FCC) has assigned 600 channels in the 800 MHz band for trunked land mobile use. Each channel is comprised of a pair of assigned frequencies, a mobile transmit frequency and a repeater transmit frequency. Mobile transmit frequencies are 806-821 MHz, with the repeater transmit frequencies exactly 45 MHz above the corresponding mobile transmit frequency, or 851-866 MHz. Channel spacing is 25 KHz, with the maximum allowed deviation between channels being .+-.5 KHz. In September 1987, the FCC also allocated 399 channels in the 900 MHz band for trunked land mobile use. Mobile transmit frequencies are 896-901 MHz, with the repeater transmit frequencies exactly 39 MHz above the corresponding mobile transmit frequency, or 935-940 MHz. Channel spacing is 12.5 KHz, with the maximum allowed deviation between channels being .+-.2.5 KHz.
In transmission trunked communication systems, a signaling protocol is used to send and receive control signals among users on each channel in the trunked system and a switching protocol is used to establish which channels those users will be communicating over. The preferred conventional transmission trunked communication system uses a signaling protocol that transmits the control signals in the subaudio band simultaneously with the transmission of voice or data information signals. Signaling protocols that can communicate control signals within the constraints of the subaudio band are preferred, because use of the subaudio band precludes the need for using a dedicated channel for transmitting the control signals (thereby reducing the number of available channels in the trunked system for voice and data communications). A switching protocol is used by the trunked system to automatically find and engage an open channel when a user initiates a transmission. To maximize the trunking capabilities of such a system, the switching protocol must efficient allocate channels in the trunked system and avoid channels that are already in use at the time the transmission is initiated. For further explanation of the preferred conventional transmission trunked communication systems, reference is made to the description of the operation of the ClearChannel LTR.RTM. system contained in the manual entitled "E. F. Johnson ClearChannel LTR Application Note", Part No. 009-0001-020 (Rev. 5, October 1988), available from E. F. Johnson Company, Waseca, Minn., which is fully incorporated by reference herein.
Transmission trunked communication systems have proven to be an economical and effective means for establishing voice and data communications between a dispatch console or control station and a fleet of mobile vehicles in a given coverage zone. However, the capability of such trunked systems to provide radio communications over a wide area serviced by a plurality of preferably adjacent coverage zones has been limited because of the problems involved in linking multiple coverage zones and because of the limitations of the switching and signaling protocols of present transmission trunked communication systems.
Present transmission trunked communication systems of the type described above are generally unable to transfer voice/data communication between coverage zones, because the repeaters in such systems are interconnected only by a single time slot status bus. As a result, the only method of interconnecting repeaters in different coverage areas is to use an external network, i.e. routing the communication as a long-distance telephone call from a repeater interconnect or Interface to a telephone exchange and then back through a second repeater interconnect to the remote repeater location. These channels are then "hung" to lock the channels in for the duration of the call. In addition to the increased expense and inconvenience of such an external network, the use of a repeater interconnect prevents usage of the channels in each trunked system over which the communication is occurring for the entire period of the communication.
Even in those instances where inter-coverage zone communications are made through an external network, such communications are for individuals calls, not fleet calls, and are limited to a few preselected users on each system. It would be desirable to provide unique ID numbers for each user on a wide area network to allow for direct entry of the unique ID to establish private communication between users on the network, as well as fleet calls between groups of users on the network. Unfortunately, it is impossible to accommodate unique ID numbers in the preferred present transmission trunked communication system because the number of digital bits required to represent such unique ID numbers exceeds the maximum number of bits that the trunked systems can communicate via the established signaling protocol. The signaling protocol of the preferred transmission trunked communication system is also limited in the number of channels that may be grouped together per trunked system. For example, the signaling protocol of the LTR.RTM. trunked system is limited to 20 channels per system and uses a look-up table to translate the channel information transmitted by the signaling protocol into the actual frequency pair assigned to that channel.
Another problem with the conventional switching and signaling protocols is that such protocols are unable to allow for the implementation of an extended feature set of radio communication capabilities on the preferred conventional transmission trunked communication systems. For example, present trunked systems are generally unable to establish priority access for users in a given coverage zone. Present trunked systems are also unable to interrogate and reprogram or otherwise modify the operation of mobiles without having the mobile transceiver unit physically brought to a service facility to be reprogrammed. A mobile traveling from one coverage zone to another coverage zone, for instance, needs to be reprogrammed for a different set of groups over which communications will be received. Such reprogramming cannot be done "on the fly" in present systems, and mobiles are therefor not able to transit coverage zones at will.
One approach to a wide area network for communications systems is cellular telephone networks. In cellular telephone networks, a large number of overlapping coverage zones (cells) are used to provide coverage over an entire area. The primary difficulties that must be overcome by cellular networks relate to the handoff of communications when a mobile transceiver moves from cell to cell during a transmission. This type of approach is unnecessary for transmission trunked communications systems because the coverage zones of trunked systems are much larger. In addition, because transmissions on a transmission trunked communication system are trunked and are relatively short, there is no need to handoff a mobile transceiver from one coverage zone to another during a transmission. Although automatic handoff of radio communications is well developed in cellular radiotelephone systems, such an automatic handoff occurs within a coordinated single system under the direction of a central processing system, not between separate coverage zones utilizing distributed processing techniques. Consequently, the body of knowledge regarding such handoff techniques is not particularly applicable when coordination between two or more trunked systems is minimal and the trunked systems are to be distributively interconnected.
Another approach to a wide area network for transmission trunked communication systems uses a separate control channel for communicating control signals and simulcasts the same message on the same frequencies for different coverage zones. This approach requires an extensive centralized processing capability to monitor and assign channels for communications extending beyond a single coverage zone. Stir another approach to a wide area network for transmission trunked communication systems uses a high speed mode to communicate extended control signals with the mobile transceivers in a coverage zone. Although the use of a high speed mode allows for large amounts of control information to be communicated with the mobile, the use of different transmission speeds may cause signal degradation and possibly missed communications because of the required switching between high speed mode and normal communications with the mobile transceiver.
The underlying problem with previous approaches to establishing a wide area network for a transmission trunked communication system is that the networks are centralized and require significant amounts of control information to be both communicated between the repeaters and the mobile (either by a dedicated control channel or by the use of a high speed mode) and be processed by a central processing unit that coordinates all of this information about each user in the entire network. Each of these systems results in increased overhead, both in terms of the hardware required to operate the trunked systems and the network itself, and in terms of the amount of control signals and other information that is communicated between the repeater and the mobile transceivers. Moreover, the use of a centralized approach to the wide area network significantly decreases the fault tolerance capabilities of the network and of the transmission trunked communication systems themselves. The reliance on a powerful central processing capability to perform the network operations leave users open to the possibility of loss of radio communication service in the event that the central processing unit fails.
Although present transmission trunked communication systems have proven to be an economical and effective means for establishing voice and data communications between a control station and a fleet of mobile vehicles in a given coverage zone, it would be advantageous to provide a method and apparatus for distributively interconnecting a plurality of land mobile transmission trunked communication systems into a wide area network that included switching and signaling protocols especially adapted for communication of control signals in the subaudio band and an expanded feature set of radio communications capabilities.